Field of the Invention
The present invention relates to a low-pass filter circuit, and a power supply device equipped with a low-pass filter circuit.
Background Art
A related art low-pass filter circuit will be described. FIG. 9 is a circuit diagram of a power supply device equipped with the related art low-pass filter circuit.
The power supply device equipped with the related art low-pass filter circuit is comprised of a feedback circuit 203, an error amplifier circuit 204, a reference voltage source 205, an output transistor 206, a resistor 211, a capacitor 212, and an NMOS transistor 213.
A first reference voltage V1 outputted from the reference voltage source 205 is converted into a second reference voltage V2 from which high frequency noise is eliminated by a low-pass filter circuit formed by the resistor 211, the capacitor 212 and the NMOS transistor 213. The feedback circuit 203 divides a voltage Vo of a constant voltage output terminal to output a feedback voltage Vfb. The error amplifier circuit 204 is inputted with the second reference voltage V2 at a non-inversion input terminal and inputted with the feedback voltage Vfb at an inversion input terminal, and controls the output transistor 206 in such a manner that the second reference voltage V2 and the feedback voltage Vfb coincide with each other. With this configuration, a low-noise voltage based on the second reference voltage V2 is outputted to a power supply output terminal.
Further, a control voltage Von is inputted to a control terminal of the NMOS transistor 213. Since a time constant of the low-pass filter circuit becomes small when the control voltage Von is high and the NMOS transistor 213 is in an ON state, the second reference voltage V2 as the output voltage of the low-pass filter circuit is started at high speed. On the other hand, since the time constant of the low-pass filter circuit becomes large when the control voltage Von is low and the NMOS transistor 213 is in an OFF state, the noise elimination capability of the low-pass filter circuit becomes high. The control voltage Von is generated by a control circuit described in, for example, FIG. 2 of Patent Document 1.
The power supply device can be realized which is started at high speed and provides a low-noise output voltage by switching the time constant of the low-pass filter circuit using the control voltage Von in this manner.
[Patent Document 1] Japanese Patent Application Laid-Open No. Hei 8 (1996)-16259